Process for preparation of di- and polyamines of the diphenylmethane series

ABSTRACT

The invention relates to a method for the preparation of di- and polyamines of the diphenylmethane series, care being taken during the shutdown of the production process that an excess of aniline and respect to formalin is ensured.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase Application of PCT/EP2015/063922,filed Jun. 22, 2015, which claims priority to European Application No.14173581.1, filed Jun. 24, 2014, each of which being incorporated hereinby reference.

FIELD

The invention relates to a process for preparing diamines and polyaminesof the diphenylmethane series, in which care is taken during therunning-down of the production process that an excess of aniline overformalin is ensured.

BACKGROUND

The continuous or partially discontinuous preparation of MDA isdisclosed in, for example, EP 1 616 890 A1, U.S. Pat. No. 5,286,760,EP-A-451442 and WO-A-99/40059. The acidic concentration of aromaticamines and formaldehyde to form diamines and polyamines of thediphenylmethane series proceeds in a plurality of reaction steps.

In the aminal process, formaldehyde is firstly condensed with aniline inthe absence of an acid catalyst to form aminal, with water beingeliminated. The rearrangement to form MDA is then carried out in thepresence of an acid catalyst in a first step to form para- andortho-aminobenzylaniline. The aminobenzylanilines rearrange in a secondstep to form MDA. Main products of the acid-catalyzed reaction ofaniline and formaldehyde are the diamine 4,4′-MDA, its positionalisomers 2,4′-MDA and 2,2′-MDA and also higher homologs.

In the neutralization process, aniline and formaldehyde are converteddirectly in the presence of an acid catalyst into aminobenzylanilineswhich subsequently react further to form the two-ring MDA isomers andMDA homologs having more than two rings.

Regardless of the process variant for preparing the acidic reactionmixture, the work-up of this is, according to the prior art, commencedby neutralization by means of a base. The neutralization is usuallycarried out at temperatures of, for example, from 90° C. to 100° C.without addition of further substances. (H. J. Twitchett, Chem. Soc.Rev. 3(2), 223 (1974)). However, it can also be carried out at adifferent temperature level, for example in order to accelerate thedegradation of interfering by-products. Hydroxides of the alkali andalkaline earth elements are suitable as bases. Preference is given tousing aqueous NaOH.

After neutralization, the organic phase is separated from the aqueousphase in a separation vessel. The organic phase containing crude MDAwhich remains after the aqueous phase has been separated off issubjected to further work-up steps, for example, a wash with water (basewash) in order to wash residual salts out from the crude MDA. The crudeMDA which has been purified in this way is finally freed of excessaniline, water, and other materials (e.g. further solvents) present inthe mixture by suitable processes such as distillation, extraction orcrystallization. The work-up which is routine according to the prior artis disclosed, for example, in EP 1 652 835 A1, page 3, line 58 to page4, line 13, or EP 2 103 595 A1, page 7, lines 21 to 37.

EP 2 486 975 A1 discloses the use of a specific mixer-reactor in thepreparation of MDA. It is stated that a local excess of formaldehyde canlead to the formation of network-like polymers. However, the patentapplication gives no details about the configuration of the reactionrunning-down procedure, i.e. interruption of the process. In particular,the patent application does not teach that the “A/F ratio” (the molarratio of aniline to formaldehyde) during the running-down procedureshould be above the A/F ratio during normal operation.

EP 1 616 890 A1 teaches that aniline and formaldehyde are firstlyreacted in the absence of the acid catalyst to form aminal and theaminal is subsequently admixed with an acid catalyst and is reactedfurther at temperatures of from 20° C. to 100° C. and at water contentsof the acidic reaction mixture obtained in this way of from 0 to 20% byweight. In particular, the water is firstly at least partly removed fromthe aminal after condensation of formaldehyde and aniline to formaminal, with a water content of from 0 to 5% by weight in the aminalbeing set, and the aminal is subsequently admixed with an acid catalystand is reacted further at temperatures of from 20° C. to 100° C. andwater contents of the acidic reaction mixture obtained in this way offrom 0 to 20% by weight. Mixtures of diamines and polyamines of thediphenylmethane series having degrees of protonation of <15%, preferablyfrom 4% to 14%, particularly preferably from 5% to 13%, can be preparedin this way. Here, the degree of protonation is, in the case ofmonoprotic acid catalysts (e.g. hydrochloric acid), the molar ratio ofthe amount of acid catalyst used and the molar amount of amine functionspresent in the reaction mixture. The patent application does not givenany details regarding the procedure during running-down of an industrialproduction plant. The examples gives are laboratory experiments. Inparticular, this patent application does not teach that the A/F ratioduring the running-down procedure should be above the A/F ratio duringnormal operation.

EP 0 283 757 A1 is likewise concerned with the preparation of MDA. Theprocess described is characterized by the addition of aniline-free MDAsto aminobenzylamines formed by condensation of aniline and formaldehydebefore the rearrangement reaction of these induced by heat. Example 2describes a mode of operation in which a small part of the MDA formed isrecirculated to the rearrangement reaction (cf. also claim 8). In otherwords: the configuration of an MDA plant in continuous normal operationis described. Details of the procedure when running-down an MDA plantare not described; in particular, there is no information about the A/Fratio during running-down compared to the A/F ratio during the reaction.

WO-A-99/40059 teaches that, in order to prepare methylenedi(phenylamine)by reaction of aniline with formaldehyde in the presence of acidcatalysts in a semicontinous process, aniline and optionally acidcatalyst are initially charged, formaldehyde and optionally acidcatalyst are fed through a mixing device into a circuit in whichaniline, optionally acid catalyst and optionally previously introducedformaldehyde are circulated, and the reaction mixture is brought to atemperature of greater than 75° C. after introduction of at least 50% ofthe total amount of formaldehyde to be fed in. The addition up to anamount of at least 50% of the total amount of formaldehyde to be fed inis carried out at a temperature of the reaction mixture in the circuitof from 20° C. to 75° C.

None of the above-mentioned document of the prior art suggests using aA/F ratio which differs from that during normal operation duringrunning-down of the reaction for preparing MDA. It is thus quite routinein the prior art to use A/F ratios during normal operation which areabove that conforming to the stoichiometry of the reaction (2:1).However, the prior art does not in any way suggest maintaining evengreater A/F ratios during running-down.

The quality of a process for preparing MDA is defined firstly by thecontent of undesirable by-products of the reaction in the product.Secondly, the quality of a process is defined by the overall processfrom start-up, normal production to running-down of the process beingable to be operated without technical failure of production or problemswhich require intervention in the process and by there being no lossesof starting materials, intermediates or end product.

Such problems can, for example occur during taking the aminal reactionout of operation (during “running-down”). Such problems can, forexample, be that there is formation of high molecular weight solidswhich lead to caking and blockages on the equipment (aminal vessel,aminal cooler and aminal separator and conduits).

Although the above-described processes of the prior art are able toproduce MDA in high yield without a reduction in quality of the endproducts, only processes which are in normal operation are described.Running-down processes until stoppage of the plant is attained (known as“running-down time”) are not taken into account.

Start-up and running-down times frequently occur in everyday industrialoperation and are not necessarily associated with opening or othermechanical intervention in a reactor or other apparatus of the plant,but can also be associated with shutting-down and restarting of theproduction plant for various other reasons, e.g. lack of raw materials.These running-down times are in practice characterized in thatdeviations in the desired molar ratio of aniline to formalin can occur.

SUMMARY

It would therefore be desirable to have an improved process forpreparing diamines and polyamines of the diphenylmethane series, inwhich attention is paid to the period of time during which the reactionis run down. It is an object of the present invention to provide such aprocess.

This object is achieved according to the invention by a process forpreparing diamines and polyamines of the diphenylmethane series (MDA) byreaction of aniline (1) and formaldehyde (2) at a molar ratio of aniline(1) to formaldehyde (2) in normal operation of A/F_(normal operation),which preferably has a value of from 1.5 to 20, particularly preferablyfrom 1.5 to 15, very particularly preferably from 1.5 to 10 and veryespecially preferably from 1.5 to 6, which comprises the steps:

Either, according to a variant A)

-   A.I) Reaction of aniline (1) and formaldehyde (2) in the absence of    an acid catalyst (3) in a reactor to form an aminal, where    aniline (1) is introduced at a mass flow rate m₁ and formaldehyde is    introduced at a mass flow rate m₂ into the reactor and the resulting    reaction mixture is subsequently separated into an aqueous phase and    an organic, aminal-containing phase;-   A.II) Reaction of at least part of the organic, aminal-containing    phase obtained in step I) with acid (3) in a reactor, with the    aminal reacting to form diamines and polyamines of the    diphenylmethane series;    wherein the following steps are carried out to end the production of    the diamines and polyamines of the diphenylmethane series:    -   A.I.1) reduction of the mass flow rate m₂ of the        formaldehyde (2) into the reactor of step A.I), commencing at a        point in time t₀, until m₂ is zero at a point in time t₁, where        t₁>t₀;    -   A.I.2) reduction of the mass flow rate m₁ of the aniline into        the reactor of step A.I) until m₁ is zero at a point in time t₂,        where t₂≧t₁;    -   A.II.1) reduction of the mass flow rate m₃ of acid (3), until m₃        zero;    -   and        the reduction of the mass flow rates m₁ and m₂ occurs in such a        way that, up to attainment of the point in time t₁, the        instantaneous molar ratio of        aniline (1) introduced into the reactor of step A.I)        to        formaldehyde (2) introduced into the reactor of step A.I),        A/F_(inst.), is always ≧1.5, preferably ≧2, where A/F_(inst.) in        all embodiments is always ≧1.05·A/F_(normal operation) up to        attainment of the point in time t₁;        or, according to a variant B),-   B.I) reaction of aniline (1) and acid (3) in a reactor to form a    reaction mixture containing the anilinium salt of the acid (3) used;-   B.II) reaction of at least part of the reaction mixture obtained in    step B.I) with formaldehyde (2) in a reactor, optionally with    introduction of further aniline (1), optionally with the    introduction of further acid (3), forming diamines and polyamines of    the diphenylmethane series;    wherein the following steps are carried out to end the production of    the diamines and polyamines of the diphenylmethane series:    -   B.II.1) reduction of the mass flow rate m₂ of formaldehyde (2)        into the reactor of step B.II) commencing at a point in time t₀,        until m₂ is zero at a point in time t₁, where t₁>t₀;    -   B.I.1) reduction of the mass flow rate m₁ of aniline into the        reactor of step B.I) and optionally into the reactor of step        B.II), until m₁ is zero at a point in time t₂, where t₂≧t₁;    -   B.I.2) reduction of the mass flow rate m₃ of the acid (3) into        the reactor of step B.I) and optionally into the reactor of step        B.II), until m₃ is zero;    -   and        the reduction of the mass flow rates m₁ and m₂ occurs in such a        way that, up to attainment of the point in time t₁, the        instantaneous molar ratio of        aniline (1) introduced into the reactor of step B.I) and, if        present, aniline (1) introduced into the reactor of step B.II)        to        formaldehyde (2) introduced into the reactor of step B.II),        A/F_(inst.), is always ≧1.5, preferably ≧2, where A/F_(inst.) in        all embodiments is particularly preferably always        ≧1.05·A/F_(normal operation) up to attainment of the point in        time t₁.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the course over time of the mass flow rates of aniline andformaldehyde in an embodiment according to variant A) of the process ofthe invention.

FIG. 2 shows, in a manner analogous to FIG. 1, the course over time ofthe mass flow rates of aniline and formaldehyde in a further embodimentaccording to variant A) of the process of the invention.

FIG. 3 shows, in a manner analogous to FIG. 2, the course over time ofthe mass flow rates of aniline and formaldehyde, with the introductionof aniline into the aminal reactor being reduced at two differentspeeds.

FIG. 4 depicts an embodiment of the variant A) in which both theintroduction of aniline and the introduction of formaldehyde into theaminal reactor are reduced at the point in time t₀ and the reduction isselected in such a way that the introduction of the two reactioncomponents into the aminal reactor is zero at the same point in time,while adhering to the boundary conditions according to the invention inrespect of the molar ratio of aniline to formaldehyde.

DETAILED DESCRIPTION

The running-down procedure can proceed from a state of full load,partial load or the minimum load specific to the plant concerned.Regardless of the prevailing load, the state from which the running-downprocedure is commenced at the point in time t₀ is, for the purposes ofthe present invention, referred to as normal operation.

For the purposes of the present invention, diamines and polyamines ofthe diphenylmethane series are amines and mixtures of amines of thefollowing type:

Here, n is a natural number of ≧2. In the following, the compounds ofthis type in which n=2 will also be referred to as diamines of thediphenylmethane series or diaminodiphenylmethanes (hereinafter MMDA).Compounds of this type in which n>2 will for the purposes of the presentinvention also be referred to as polyamines of the diphenylmethaneseries or polyphenylenepolymethylenepolyamine (hereinafter PMDA).Mixtures of the two types will also be referred to as diamines andpolyamines of the diphenylmethane series (hereinafter MDA).Industrially, the diamine and polyamine mixtures are predominantlyconverted by phosgenation into the corresponding diisocyanates andpolyisocyanates of the diphenylmethane series.

In both variants, the reactors of steps I) and II) can be identical ordifferent. This means that in variant A) it is equally possible for theaminal formed in step A.I) to be left in the reactor and the acid to beadded or for the aminal to be transferred into another reactor and theacid (3) then to be added there. In variant B), it is equally possiblefor the reaction product of aniline (1) and acid (3) formed in step B.I)to be left in the reactor and the formaldehyde (2) to be added or forthe reaction product of aniline (1) and acid (3) to be transferred toanother reactor and the formaldehyde (2) then to be added there.Furthermore, the term “a reactor” also encompasses, for the purposes ofthe present invention, the case that a reactor cascade is used (in otherwords the word “a” is in this context to be interpreted as theindefinite article and not as an indication of number).

In both variants, the steps I) and II) are carried out continuously orsemicontinuously, preferably continuously.

The instantaneous molar ratio, A/F_(inst.), in the period of time up tot₁ can in the case of variant A) be derived in a simple manner from theknown feed streams (1) and (2) into the reactor of step A.I) at aparticular point in time t. In the case of variant B), the instantaneousmolar ratio, A/F_(inst.), in the period of time up to t₁ can be derivedin an analogous way from the known feed streams (1) and (2) into thereactor of step B.I) or into the reactor of step B.II) at a particularpoint in time t. If use is made of the possibility of introducingfurther aniline into the reactor of step B.II) in step B.II), this is,for the purpose of determining the instantaneous molar ratio,A/F_(inst.), added during the period of time up to t₁ to the aniline ofstep B.I). If such aniline added in step B.II) is mixed beforehand withacid so that it is present as anilinium salt, this does not change thecalculation since one mole of aniline reacts with one mole of acid toform one mole of anilinium salt. For the purposes of calculating theinstantaneous molar ratio A/F_(inst.), the calculation can be carriedout as if all aniline (1) were present in free form.

For the purposes of the invention, the reduction of the mass flow ratesm₁ and m₂ (for example expressed in kg of feed aniline/hour and kg offormaldehyde/hour or kg formaldehyde solution/hour) is carried out insuch a way that A/F_(inst.) is always ≧1.5, preferably ≧2, until thepoint in time t₁ has been attained, with A/F_(inst.) particularlypreferably being ≧1.05·A/F_(normal operation) up to attainment of thepoint in time t₁ in all embodiments. For theoretical reasons, this doesnot apply at the point in time t₁ itself since the mass flow rate m₂ ofthe formaldehyde is then zero. In any case, however, the reduction ofthe formaldehyde input is carried out in such a way that at least thestoichiometric amount of aniline required for formation of MDA ispresent in the respective reactor during the running-down procedure.

In both variants, the introduction of formaldehyde into the respectivereactor is, preferably steplessly, throttled back to zero during theperiod of time from t₀ to t₁ in the first step of the running-downprocedure (step A.I.1) or B.II.1)), while the introduction of anilinecontinues to run at the same load, a greater load or a reduced load.This allows complete reaction of the formaldehyde present in the reactorof step A.I) or B.II) and discharge of the product formed from thisreactor until, in variant A, virtually only aniline is present in theaminal reactor and the apparatuses of the further process steps, or, invariant B, virtually only aniline is present in the reactor of stepB.II) and in the apparatuses of the further process steps.

In the second step of the running-down procedure (step A.I.1) orB.I.1)), the introduction of aniline is finally stopped.

In the third step of the running-down procedure (step A.II.1) orB.I.2)), the introduction of acid is stopped. In both variants, the massflow rate m₃ of acid (3) is preferably reduced commencing at theearliest at the point in time t₁. In both variants, particularpreference is given to maintaining the mass flow rate m₃ prevailing atthe commencement of the running-down procedure (i.e. before steps A.I.1)or B.II.1)) are carried out) up to the point in time t₂ and thenstarting the introduction of further acid (3) (for example by carefullyclosing a valve), so that any residual amounts of acid (3) remaining inthe feed conduits continue to be fed in for a certain time. This ensuresthat sufficiently large amounts of salt are present in the subsequentwork-up (neutralization), which make phase separation easier.

Embodiments of the process of the invention are described below. Theycan be combined with one another in any way, unless the contrary isclear from the context.

During the running-down procedure, the reaction temperature in thereactor space of the reactor of step A.I) is, for example, from 20° C.to 120° C., preferably from 40° C. to 110° C. and very preferably from60° C. to 100° C., in the case of variant A). The reactor of step A.I)(aminal reactor) is operated at atmospheric pressure or atsuperatmospheric pressure. Preference is given to a pressure of from1.05 to 5 bar absolute, very particularly preferably from 1.1 to 3 barand very particularly preferably from 1.2 bar to 2 bar absolute,prevailing. The pressure is maintained by means of pressure regulatingvalves or by connecting the offgas systems of the aminal reactor and theoverflow of the aminal separator. The aminal separator and the outflowof the aqueous phase are preferably heated in order to prevent caking.

In the case of variant B), the reaction temperature in the reactor spaceof the reactor of step B.II) is, for example, from 20° C. to 200° C. andpreferably from 20° C. to 160° C. during the running-down procedure.Before running-down, the temperature is >75° C., and the temperaturedecreases during running-down if the reactor is not heated.

After running-down, the apparatuses with aniline still present thereincan be allowed to stand. As an alternative, one apparatus, more than oneapparatus or all apparatuses can be emptied, for example in order toprepare them for maintenance measures.

According to the invention, the reduction of the mass flow rates m₁ andm₂ (for example expressed in kg of feed aniline/hour and kg offormaldehyde/hour or kg of formaldehyde solution/hour) in steps I.1) andI.2) is carried out in such a way that the molar ratio of aniline toformaldehyde in the first reactor is ≧2:1 up to attainment of the pointin time t₁. For theoretical reasons, this does not apply at the point intime t₁ itself since the mass flow rate m₂ of the formaldehyde is thenzero. However, the reduction in the formaldehyde input is in any casecarried out so that at least the stoichiometric amount of anilinerequired for formation of MDA is present in the reactor.

If two or more MDA reactor lines are to be operated in parallel, it ispossible for one reactor line firstly to be run down and the otherreactor lines to be run down in succession, but this does not have to bethe case. It is also possible to run down all MDA reactor lines close tosimultaneously.

In a further embodiment of the process of the invention, the mixtureobtained is also discharged from the reactor during and/or after stepA.I) or B.II). As a result, the production process including theparticipating transport of materials can be continued. The discharge isadvantageously continued until, in variant A, virtually only aniline ispresent in the aminal reactor and in the apparatuses of the furtherprocess steps or, in variant B, virtually only aniline is present in thereactor of step B.II) and in the apparatuses of the further processsteps.

In a further embodiment of the process of the invention, t₂−t₁>0 hour inboth variants A) and B). This period of time is preferably from >0 hourto <30 hours, more preferably from >0.5 hour to <10 hours and veryparticularly preferably from >1 hour to <5 hours.

In a further embodiment of the process of the invention, the mass flowrate m₁ is ≧1000 kg/hour in both variants A) and B). This mass flow rateis preferably from ≧2000 kg/hour to ≦200 000 kg/hour, more preferablyfrom ≧3000 kg/hour to ≦100 000 kg/hour.

In a further embodiment of the process of the invention, the mass flowrate m₂ is ≧300 kg/hour in step I) and in step II) in both variants A)and B). This mass flow rate is preferably from ≧400 kg/hour to ≦100 000kg/hour, more preferably from ≧500 kg/hour to ≦50 000 kg/hour.

The formaldehyde (2) used can in both variants originate from all knownproduction processes for formaldehyde. Mention will merely be made byway of example of the silver catalyst process.

In a further embodiment of the process of the invention, at least thereactor of step A.I) or of step B.I) is, in both variants A) and B), atleast partly filled with aniline after the point in time t₂; i.e.aniline (or anilinium salt) still present in the respective reactorafter ending of the introduction of aniline is not completelydischarged. When aniline (or anilinium salt) remains in the respectivereactor without the presence of formaldehyde, production can beinterrupted without formation of undesirable high molecular weightby-products.

The ways in which the process is carried out in the variants A) and B)in normal operation up to the point at which the crude product isobtained are described in more detail below:

The preparation of diamines and/or polyamines of the diphenylmethaneseries according to variant A) can be summarized by way of example asfollows:

a) key procedure of step I): aniline and formaldehyde are condensed inthe absence of an acid catalyst to form aminal and water and theresulting aminal is discharged from the aminal reactor, andb) water from step a), which originates mainly from water ofcondensation of the aminal reaction and water from the starting materialformaldehyde, is at least partly separated off as an aqueous phase fromthe reaction mixture from the aminal reaction, andc) key procedure of step II): the aminal from step b) is rearranged inthe presence of an acid catalyst to form MDA.

The condensation of aniline and formaldehyde in step a) can be carriedout by any method according to the prior art. Here, aniline and aqueousformaldehyde solution are normally condensed in molar ratios in therange from 1.5 to 20, preferably from 1.5 to 15, particularly preferablyfrom 1.5 to 10 and very particularly preferably from 1.5 to 6, attemperatures of from 20° C. to 120° C., preferably from 40° C. to 110°C. and particularly preferably from 60° C. to 100° C., to form aminaland water. The reaction is usually carried out at atmospheric pressure.Suitable aniline grades are, for example, described in EP 1 257 522 B1,EP 2 103 595 A1 and EP 1 813 598 B1. Preference is given to usingtechnical grades of formalin (aqueous solution of formaldehyde)containing from 30% by mass to 50% by mass of formaldehyde in water.However, formaldehyde solutions having lower or higher concentrations orthe use of gaseous formaldehyde are also conceivable.

In step b), organic aminal phase and aqueous phase are separated attemperatures of from 20° C. to 120° C., preferably from 40° C. to 110°C., particularly preferably from 60° C. to 100° C., preferably atambient pressure. The phase separation can also be carried out atslightly superatmospheric pressure.

The rearrangement of the aminal in step c) is carried out in thepresence of an acid catalysts usually a strong mineral acid such ashydrochloric acid. Preference is given to using mineral acid in a molarratio of mineral acid to aniline of from 0.001 to 0.9, preferably from0.05 to 0.5. It is naturally also possible to use solid, acid catalystsas described in the literature. Here, formaldehyde can be introducedinto a mixture of aniline and acid catalyst and the reaction solutioncan be reacted to completion by stepwise heating. As an alternative,aniline and formaldehyde can also firstly be prereacted and subsequentlyadmixed, with or without prior removal of water, with the acid catalystor a mixture of further aniline and acid catalyst, after which thereaction solution is reacted to completion by stepwise heating. Thisreaction can be carried out continuously or batchwise by one of thenumerous methods described in the literature (e.g. in EP 1 616 890 A1 orEP 127 0544 A1).

The preparation of the crude diamines and/or polyamines of thediphenylmethane series according to variant B) can be summarized by wayof example as follows:

a) key procedure of step B.I): aniline and acid are reacted in theabsence of formaldehyde to form a reaction mixture containing theanilinium salt of the acid used, andb) key procedure of step B.II): the reaction mixture from step a), whichcontains the anilinium salt of the acid used, is admixed withformaldehyde and rearranged to form MDA.

The reaction of aniline and acid, preferably hydrochloric acid, in stepa) can be carried out by a method according to the prior art. Thefurther description will be given for the example of aqueoushydrochloric acid, but other acids can also be used. Aniline and anaqueous hydrochloric acid are normally reacted at molar ratios ofaniline to acid in the range from 1.6 to 100, preferably from 3.3 to 20.This reaction can be carried out in an upstream reactor or a mixingsection, with the reaction mixture optionally being able to betemporarily stored in a stock vessel. This reaction can optionally becarried out in the same reactor in which the subsequent reaction of thereaction mixture of aniline and acid with formaldehyde takes place.Suitable aniline grades are, for example, described in EP 1 257 522 B1,EP 2 103 595 A1 and EP 1 813 598 B1. Suitable hydrochloric acid gradesare, for example, described in EP 1 652 835 A1.

“Feed aniline” can firstly be placed in the reactor at temperatures offrom 10° C. to 60° C. The feed aniline is made up of fresh aniline andoptionally aniline from the MDA distillation (described in more detailfurther below; see step h)) and optionally aniline from the wastewatertreatment.

Then, for example while the introduction of aniline is proceeding, thehydrochloric acid is introduced into the initially charged aniline, withcare being taken to ensure good mixing. This good mixing can be achievedby stirring by means of a stirrer or by circulation (by means of pumps)of the reaction mixture or by a combination of stirring and circulation.The entire plant from feed streams to offtake of product shouldoptionally be ready for operation. The reaction apparatus can, ifnecessary, be equipped with an internal or external heat exchanger inorder to be able to remove the heat of reaction evolved. As analternative, the feed aniline and/or the hydrochloric acid can also beappropriately cooled. A further alternative is the use of evaporativecooling for removing the heat of reaction.

In step b), the aniline hydrochloride-containing reaction mixture fromstep a) is reacted with aqueous formaldehyde solution. Here,formaldehyde can be introduced into a mixture of aniline and acidcatalyst and the reaction solution can be reacted to completion bystepwise heating, as described, for example, in EP 1 053 222 A1. Thereaction is usually carried out at temperatures of from 20° C. to 150°C.

The reactor of step B.I) and the reactor of step B.II) areadvantageously different from one another. However, carrying out stepsB.I) and B.II) in the same reactor is not ruled out. This reaction canbe carried out continuously, semicontinuously or batchwise.

Preference is given to use technical grades of formalin (aqueoussolution of formaldehyde) containing from 30% by mass to 50% by mass offormaldehyde in water. However, formaldehyde solutions having lower orhigher concentrations or the use of gaseous formaldehyde are alsoconceivable.

In the case of a semicontinuous or batchwise reaction, an “infinite”excess of aniline in the form of free aniline and aniline hydrochlorideis present at the point in time t₁ (=mass flow rate of formaldehydeequal to zero). From the point in time t₀ to attainment of the point intime t₁, formaldehyde is preferably introduced in such amounts that atleast 1.05 times the A/F_(normal operation) ratio provided in theformulation for normal operation is maintained.

In both variants A) and B), a crude reaction mixture containing diaminesand polyamines of the diphenylmethane series is obtained (in variant A)in step c) and in variant B) in step b)). The work-up of this reactionmixture is preferably carried out as follows, regardless of whethervariant A) or B) is employed:

d) the reaction mixture containing diamines and polyamines of thediphenylmethane series is neutralized, ande) the neutralized reaction mixture containing diamines and polyaminesof the diphenylmethane series is separated in a separation vessel intoan organic phase containing diamines and polyamines of thediphenylmethane series and an aqueous phase, andf) the organic phase containing diamines and polyamines of thediphenylmethane series is purified further by means of washing liquid ina washing vessel, andg) the resulting mixture is separated in a separation vessel into anorganic phase containing diamines and polyamines of the diphenylmethaneseries and an aqueous phase, andh) the washed organic phase containing diamines and polyamines of thediphenylmethane series is freed of water and aniline by distillation.

In step d), the reaction mixture containing the diamines and polyaminesof the diphenylmethane series is neutralized, optionally with additionof water and/or aniline. According to the prior art, the neutralizationis usually carried out at temperatures of, for example, from 90° C. to100° C. without addition of further substances. However, it can also becarried out at a different temperature level, for example in order toaccelerate the degradation of interfering by-products. The hydroxides ofthe alkali elements and alkaline earth elements, for example, aresuitable as bases. Aqueous NaOH is preferably employed. The base usedfor neutralization is preferably used in amounts of greater than 100%,particularly preferably from 105% to 120%, of the stoichiometricallyrequired amount for neutralization of the acid catalyst used (see EP 1652 835 A1).

The neutralized reaction mixture containing the diamines and polyaminesof the diphenylmethane series is subsequently separated in step e) intoan organic phase containing diamines and polyamines of thediphenylmethane series and an aqueous phase. This can be assisted by theaddition of aniline and/or water. If the phase separation is assisted byaddition of aniline and/or water, the addition of these/this ispreferably carried out with intensive mixing as early as in theneutralization. Here, mixing can be effected in mixing sections havingstatic mixers, in stirred vessels or cascades of stirred vessels or elsein a combination of mixing sections and stirred vessels. The reactionmixture which has been neutralized and diluted by addition of anilineand/or water is then preferably fed into an apparatus which, owing toits configuration and/or internals, is particularly suitable forseparation into an organic phase containing MDA and an aqueous phase,preferably phase separation or extraction apparatuses according to theprior art, as are described, for example, in Mass-Transfer Operations,3rd Edition, 1980, McGraw-Hill Book Co, pp. 477 to 541, or Ullmann'sEncyclopedia of Industrial Chemistry (Vol. 21, Liquid-Liquid Extraction,E. Müller et al., pages 272-274, 2012 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim, DOI: 10.1002/14356007.b03_06.pub2) or in Kirk-OthmerEncyclopedia of Chemical Technology (see“http://onlinelibrary.wiley.com/book/10.1002/0471238961”, publishedonline: Jun. 15, 2007, pages 22-23) (mixer-settler cascades or settlingvessels).

Washing of the organic phase with water follows in step f), and renewedsettling out of the water phase to remove residual contents of saltfollows in step g) (preferably as described in DE-A-2549890, page 3).

In step h), water and aniline are separated off by distillation from theorganic phase containing diamines and polyamines of the diphenylmethaneseries obtained in step g), as described in EP 1 813 597 B1. The organicphase obtained in step g) preferably has a composition, based on theweight of the mixture, of 5-15% by weight of water and, depending on theratios of aniline and formaldehyde used, 5-90% by weight, preferably5-40% by weight, of aniline and 5-90% by weight, preferably 50-90% byweight, of diamines and polyamines of the diphenylmethane series. Afterexit from the phase separation in step g), the organic phase containingdiamines and polyamines of the diphenylmethane series usually has atemperature of 80° C.-150° C.

The resulting diamines and polyamines of the diphenylmethane series canbe converted into the corresponding diisocyanates and polyisocyanates ofthe diphenylmethane series, viz MDI, by the known methods under inertconditions by reaction with phosgene in an organic solvent. Here, thephosgenation can be carried out by one of the methods known from theprior art (e.g. DE-A-844896 or DE-A-19817691).

If the above-mentioned conditions are adhered to during running-down,the following advantages are obtained:

i) avoidance of blockages and deposits in the aminal vessel, cooler,separator and the aminal cooling circulation pump and thus avoidance ofa second start-up procedure because the plant does not have to be rundown again and opened for the purpose of cleaning the equipment.ii) saving of energy because the start-up procedure does not have to becarried out a second time because of the formation of blockages anddeposits and the resulting shutdown of the plant for the purpose ofcleaning the equipment.iii) an increase in the productivity of the plant because the on-streamtimes of the reactor are increased because cleaning times for removingblockages and deposits are dispensed with.iv) avoidance or reduction of precipitates, caked material and blockagesin the equipment (aminal vessel, cooler, separator and the aminalcooling circulation pump) and, associated therewith, an increase in theoperating time of the process.v) reduced waste after cleaning the equipment (high molecular weightsolids) and saving of incineration costs.vi) avoidance of out-of-specification product which can be formed as aresult of multiple poor starting up and running down: such poor-qualityrunning-down product thus does not have to be blended with good-qualityMDA or, in the worst case, even incinerated.vii) better phase separation of aqueous and organic phases due to theabsence of high molecular weight compounds which have an adverse effecton the phase separation.

The present invention is illustrated in detail with the aid of thefollowing drawings and examples, but without being restricted thereto.

The drawings show:

FIG. 1-4 the course over time of the mass flow rates of aniline andformaldehyde in the process of the invention according to variant A)

FIG. 1 shows the course over time of the mass flow rates of aniline andformaldehyde in an embodiment according to variant A) of the process ofthe invention. The time t is plotted on the x axis and mass flow rates mare plotted on the Y axis. Until attainment of the point in time t₀, themass flow rates for aniline (m₁, in the figure denoted by “A”) and forformaldehyde (m₂, in the figure denoted by “F”) into the aminal reactordesignated in the terminology of the present invention as reactor ofstep A.I) are constant. It is now decided that the production needs tobe run down. For this purpose, the mass flow rate of formaldehyde intothe aminal reactor is reduced at an unchanged magnitude of the mass flowrate of aniline, until the mass flow rate of formaldehyde is zero at thepoint in time t₁.

At the point in time t₁, reduction of the mass flow rate of aniline intothe aminal reactor is commenced, until it is likewise zero at the pointin time t₂. It can be seen that the mass flow rate of aniline introducedinto the aminal reactor is at least so much greater than the mass flowrate of formaldehyde that the molar ratio of aniline to formaldehyde isat least 2 at any point in time during running-down of the reaction.

During the time between t₁ and t₂, the formaldehyde still present in theaminal reactor can react to completion with the aniline. Finally, therecan no longer be any free formaldehyde present in the aminal reactor.

It is of course possible in this variant for the reaction mixtureobtained to continue to be discharged from the aminal reactor after thepoints in time t₀, t₁ and/or t₂.

FIG. 2 shows, in a manner analogous to FIG. 1, the course over time ofthe mass flow rates of aniline and formaldehyde in a further embodimentaccording to variant A) of the process of the invention. Here, the massflow rate of aniline is also reduced as early as the point in time t₀.

FIG. 3 shows, in a manner analogous to FIG. 2, the course over time ofthe mass flow rates of aniline and formaldehyde, with the introductionof aniline into the aminal reactor being reduced at two differentspeeds.

FIG. 4 finally depicts an embodiment of the variant A) in which both theintroduction of aniline and the introduction of formaldehyde into theaminal reactor are reduced at the point in time t₀ and the reduction isselected in such a way that the introduction of the two reactioncomponents into the aminal reactor is zero at the same point in time,while adhering to the boundary conditions according to the invention inrespect of the molar ratio of aniline to formaldehyde.

EXAMPLES General Conditions for the Preparation of MDA in a Run-inProduction Plant (Before Commencement of the Running-Down Procedure)

In a continuous reaction process (step a)), 24.3 t/h of feed aniline(containing 90% by mass of aniline) and 9.9 t/h of 32% strength aqueousformaldehyde solution (molar ratio of aniline to formalin 2.1:1) weremixed and reacted in a stirred reaction vessel at 90° C. and 1.4 barabsolute to form aminal. The reaction vessel was provided with a coolerhaving a cooling circulation pump. The reaction mixture leaving thereaction vessel was fed into a phase separation apparatus (aminalseparator) (step b)).

After the phase separation to remove the aqueous phase, the organicphase was admixed in a mixing nozzle with 30% strength aqueoushydrochloric acid (degree of protonation 10%, i.e. 0.1 mol of HCl areadded per mole of amino groups) and fed into the first rearrangementreactor. The rearrangement reaction was carried out at from 45° C. to165° C. in a reactor cascade (step c)).

After the reaction was complete, the reaction mixture obtained wasadmixed with 32% strength sodium hydroxide solution in a molar ratio ofsodium hydroxide to HCl of 1.1:1 and reacted in a stirred neutralizationvessel (step d)). The temperature was 115° C. The absolute pressure was1.4 bar. The neutralized reaction mixture was subsequently separated ina neutralization separator into a lower, aqueous phase, which isconveyed to a wastewater collection vessel, and an organic phase (stepe)).

The upper, organic phase was conveyed to washing and washed withcondensate in a stirred washing vessel (step f)). After the washingwater had been separated off in a washing water separator (step g)), thecrude MDA obtained in this way was freed of water and aniline bydistillation, giving 17 t/h of MDA as bottom product (step h)).

Example 1 (Comparative Example): Running-Down of the MDA Plant, withAniline being Stopped First

The MDA plant was, as described in the general conditions for thepreparation of MDA, operated at a load of 17 t/h of MDA. After atechnical defect in the rearrangement reactors, aniline was stopped inthe aminal reactor and formalin was stopped two minutes later. Theaddition of hydrochloric acid was interrupted at the same point in timeas the addition of aniline. The aminal reactor and the rearrangementreactors were emptied. After the defect in the rearrangement reactorshad been rectified, the empty aminal reactor was filled with feedaniline (containing 90% by mass of aniline) until aniline flowed overvia the siphon into the aminal separator. When the aminal reactor hadbeen filled to this extent, the feed aniline flowed at a rate of 12.2t/h into the stirred aminal reaction vessel, which corresponded to 50%of the nominal load.

The formalin conduit was then opened. The reaction commenced immediatelyand the reaction mixture was regulated to 90° C. The pressure in theaminal reactor was 1.4 bar absolute during the start-up phase. Theamount of 32% strength aqueous formaldehyde solution which was to beintroduced into the aminal reactor during a planned start-up time t of45 minutes should be increased steplessly from 0 t/h to 4.95 t/h, whichcorresponded to 50% of the nominal load. However, the plant had to beshut down after 30 minutes because the aminal vessel, the aminal coolerand the aminal separator were blocked and the aminal cooling circulationpump was likewise blocked with solid and did not operate.

Example 2 (Comparative Example): Running-Down of the MDA Plant, with theIntroduction of Formalin and Aniline being Reduced Close toSimultaneously

The MDA plant was, as described in the general conditions for thepreparation of MDA, operated at a load of 17 t/h of MDA. After atechnical defect in the aminal reactor, aniline and formaldehyde werestopped at essentially the same time. The addition of hydrochloric acidwas interrupted at the same point in time as the addition of aniline.

The aminal reactor was emptied. After the defect had been rectified, theempty aminal reactor was filled with feed aniline (containing 90% bymass of aniline) until aniline flowed over via the siphon into theaminal separator. When the aminal reactor had been filled to thisextent, the feed aniline flowed at a rate of 12.2 t/h into the stirredaminal reaction vessel, which corresponded to 50% of the nominal load.

The formalin conduit was then opened. The reaction commenced immediatelyand the reaction mixture was regulated to 90° C. The pressure in theaminal reactor was 1.4 bar absolute during the start-up phase. During astart-up time of 45 minutes, the amount of 32% strength aqueousformaldehyde solution introduced into the aminal reactor was increasedsteplessly from 0 t/h to 4.95 t/h. The reaction mixture was subsequentlyconveyed from the aminal reactor into a phase separation apparatus inwhich the water of reaction from the aminal reaction was separated off.The remaining organic phase was then pumped into the first rearrangementtank, with a 30% strength aqueous hydrochloric acid, corresponding to adegree of protonation of 10% (i.e. 0.1 mol of HCl are added per mole ofamino groups) being introduced at the same time via a mixing nozzle intothe inlet for the aminal into the first rearrangement tank.

The rearrangement reaction took place at from 50° C. to 150° C. in areactor cascade (step c)). After the reaction was complete, the reactionmixture obtained was worked up as described in the general conditionsfor the preparation of MDA. After three production days, the aminalreactor had to be taken out of operation because solid had precipitatedin the aminal separator and made cleaning necessary.

Example 3 (According to the Invention): Running-Down of the MDA Plant,with Further Aniline being Fed in

The MDA plant was, as described in the general conditions for thepreparation of MDA, operated at a load of 17 t/h of MDA. After atechnical defect in the rearrangement reactors, formalin was stopped inthe aminal reactor and aniline was stopped one hour later. The additionof hydrochloric acid was interrupted at the same point in time as theaddition of aniline.

The aminal reactor and the rearrangement reactors were emptied. Afterthe defect in the rearrangement reactors had been rectified, the emptyaminal reactor was filled with feed aniline (containing 90% by mass ofaniline) until aniline flowed over via the siphon into the aminalseparator. When the aminal reactor had been filled to this extent, thefeed aniline flowed at a rate of 12.2 t/h into the stirred aminalreaction vessel, which corresponded to 50% of the nominal load.

The formalin conduit was then opened. The reaction commenced immediatelyand the reaction mixture was regulated to 90° C. The pressure in theaminal reactor was 1.4 bar absolute during the start-up phase. During astart-up time of 45 minutes, the amount of 32% strength aqueousformaldehyde solution introduced into the aminal reactor was increasedsteplessly from 0 t/h to 4.95 t/h.

The reaction mixture was subsequently conveyed from the aminal reactorinto a phase separation apparatus in which the water of reaction fromthe aminal reaction was separated off. The remaining organic phase wasthen pumped into the first rearrangement tank, with a 30% strengthaqueous hydrochloric acid, corresponding to a degree of protonation of10% (i.e. 0.1 mol of HCl are added per mole of amino groups), wasintroduced at the same time via a mixing nozzle into the inlet for theaminal into the first rearrangement tank. The rearrangement reactiontook place at from 50° C. to 150° C. in a reactor cascade (step c)).

After the reaction was complete, the reaction mixture obtained wasworked up as described in the general conditions for the preparation ofMDA.

In the mode of operation according to the invention, the formation ofdeposits in the aminal vessel, in the aminal cooler and in the aminalseparator, which can then grow further after the subsequent starting-upof the plant and during subsequent continuous operation and even after ashort time cause blockages in the aminal vessel, in the aminal cooler,in the aminal separator and also stoppage of the cooling circulationpump due to solid deposits, was prevented during the running-down phase.

In the case of correct running-down of the plant and correct subsequentstart-up of the plant, the aminal vessel could be operated withoutproblems over a long production cycle of a number of months. Theformation of undesirable by-products such as insoluble polymeric amines,etc., was significantly reduced and later blending of the start-upproduct with pure MDA or in the worst case incineration of the start-upproduct could be dispensed with.

1. A process for preparing diamines and polyamines of thediphenylmethane series (MDA) by reaction of aniline (1) and formaldehyde(2) as a molar ratio of aniline (1) to formaldehyde (2) in normaloperation of A/F_(normal operation), which comprises the steps: Either,according to a variant A) A.I) reacting aniline (1) and formaldehyde (2)in the absence of an acid catalyst (3) in a reactor to form an aminal,where aniline (1) is introduced at a mass flow rate m₁ and formaldehyde(2) is introduced at a mass flow rate m₂ into the reactor and theresulting reaction mixture is subsequently separated into an aqueousphase and an organic, aminal-containing phase; and A.II) reacting atleast part of the organic, aminal-containing phase obtained in step A.I)with acid (3) in a reactor, with the aminal reacting to form diaminesand polyamines of the diphenylmethane series; wherein the followingsteps are carried out to end the production of the diamines andpolyamines of the diphenylmethane series: A.I.1) reducing the mass flowrate m₂ of the formaldehyde (2) into the reactor of step A.I),commencing at a point in time t₀, until m₂ is zero at a point in timet₁, where t₁>t₀; A.I.2) reducing the mass flow rate m₁ of the anilineinto the reactor of step A.I) until m₁ is zero at a point in time t₂,where t₂≧t₁; and A.II.1) reducing the mass flow rate m₃ of acid (3),until m₃ zero; and the reduction of the mass flow rates m₁ and m₂ occursin such a way that, up to attainment of the point in time t₁, theinstantaneous molar ratio of aniline (1) introduced into the reactor ofstep A.I) to formaldehyde (2) introduced into the reactor of step A.I),A/F_(inst.), is always ≧1.5; or, according to a variant B), B.I)reacting aniline (1) and acid (3) in a reactor to form a reactionmixture containing the anilinium salt of the acid (3) used; and B.II)reacting at least part of the reaction mixture obtained in step B.I)with formaldehyde (2) in a reactor, optionally with introduction offurther aniline (1), optionally with the introduction of further acid(3), forming diamines and polyamines of the diphenylmethane series;wherein the following steps are carded out to end the production of thediamines and polyamines of the diphenylmethane series: B.II.1) reducingthe mass flow rate m₂ of formaldehyde (2) into the reactor of step B.II)commencing at a point in time t₀, until m₂ is zero at a point in timet₁, where t₁>t₀; B.I.1) reducing the mass flow rate m₁ of aniline intothe reactor of step B.I) and optionally into the reactor of step B.II),until m₁ is zero at a point in time t₂, where t₂≧t₁; and B.I.2) reducingthe mass flow rate m₃ of the acid (3) into the reactor of step B.I) andoptionally into the reactor of step B.II), until m₃ is zero; and thereduction of the mass flow rates m₁ and m₂ occurs in such a way that, upto attainment of the point in time t₁, the instantaneous molar ratio ofaniline (1) introduced into the reactor of step B.I) and, if present,aniline (1) introduced into the reactor of step B.II) to formaldehyde(2) introduced into the reactor of step B.II), A/F_(inst.), is always≧1.5.
 2. The process as claimed in claim 1, wherein the steps I) and II)of both variants are carried out in the form of a continuous process. 3.The process as claimed in claim 1, wherein the mixture obtained is alsodischarged from the respective reactor during and/or after step A.I) orB.II).
 4. The process as claimed in claim 1, wherein t₂−t₁ is >0 hour inboth variants.
 5. The process as claimed in claim 1, wherein the massflow rate m₁ is ≧1000 kg/hour in both variants.
 6. The process asclaimed in claim 1, wherein the mass flow rate m₂ is ≧300 kg/hour inboth variants.
 7. The process as claimed in claim 1, wherein, in bothvariants, at least the reactor of step A.I) or the reactor of step B.I)is at least partly full of aniline after the point in time t₂.
 8. Theprocess as claimed in claim 1, wherein the mass flow rate m₃ of acid (3)is reduced at the earliest commencing at the point in time t₁ in bothvariants.
 9. The process as claimed in claim 8, wherein, in bothvariants, the mass flow rate m₃ prevailing before steps A.I.1) orB.II.1) are carried out is maintained up to the point in time t₂. 10.The process as claimed in claim 8, wherein the introduction of furtheracid (3) is stopped at the point in time t₂ in both variants.
 11. Theprocess of claim 1, wherein, according to variant A, the reduction ofthe mass flow rates m₁ and m₂ occurs in such a way that, up toattainment of the point in time t₁, the instantaneous molar ratio ofaniline (1) introduced into the reactor of step A.I) to formaldehyde (2)introduced into the reactor of step A.I), A/F_(inst.), is always≧1.05·A/F_(normal operation).
 12. The process of claim 1, whereinaccording to variant B), the reduction of the mass flow rates m₁ and m₂occurs in such a way that, up to attainment of the point in time t₁, theinstantaneous molar ratio of aniline (1) introduced into the reactor ofstep B.I) and, if present, aniline (1) introduced into the reactor ofstep B.II) to formaldehyde (2) introduced into the reactor of stepB.II), A/F_(inst.), is always ≧1.05·A/F_(normal operation.)